The long-range goal of this work is to determine how B cell tolerance to self-antigens in systemic lupus erythematosus (SLE) is lost. The focus will be the response to the nuclear antigen Smith (Sm), which is unique to human and mouse SLE. We have shown that in non-autoimmune mice some anti-Sin B cells are regulated by negative selection (anergy, developmental arrest, central deletion), while others are positively selected into the marginal zone and B- 1 subsets and are functional. This coexistence of negatively and positively selected B cells is unusual and suggests a possible model for the anti-Sm response. The hypothesis to be tested is that one or few positively selected anti-Sm B cells are activated initially, and that the antibody they produce activates additional anti-Sm B cells, including those that are negatively selected. In Aim 1 we will determine which mechanism(s) of anti-Sm B cell regulation are defective in autoimmune MRL and lpr mice by generating a series Ig H and L chain transgenic mice regulated by different mechanism. These mice will be followed for anti-Sin B cell activation to identify the mechanism(s) activated. In Aim 2 we will determine whether the repertoire of anti-Sm B cells involved in the response expands during its course to include a larger repertoire of B cell clones. Whether anti-Sm antibodies generated early in the response can activate other anti-Sin B cells will also be determined. In Aim 3 we will examine the anti-Sm response in human SLE. We can detect anti-Sm B cells in the peripheral blood of SLE patients and find that they express unusually high CD19 levels, although non-Sm binding na'fve cells have unusually low CD19 levels. We will test the hypothesis that the anti-Sm response in human SLE is antigen-driven and that intra-clonal diversity and affinity maturation are additive through successive periods of active disease. In addition, we will test the hypothesis that the unusual pattern of CD 19 expression affects tolerance and activation.